LONDON (AP) Double-amputee runner Oscar Pistorius hasn't been able to skip one of the games' biggest bureaucratic hurdles: the Paralympics disability classification system.

It aims for fair competition, but with so many disabilities and degrees of severity, classifying Paralympic athletes can often seem like a sport in itself. According to that system, Pistorius is classed as a double amputee below the knee and eligible only to compete against other Paralympic athletes with a similar impairment.

The South African recently made history as the first disabled athlete to compete on the track at the Olympics, running in the 400 meters on carbon-fiber blades.

"We assume that Oscar is worse off than someone who has only lost one limb, but in fact he is a very balanced runner," said David Howe, a disability sport expert at Britain's Loughborough University and former Paralympic runner. "Someone who is running with one human leg and one prosthetic will be more unbalanced."

The classification, while "reasonably good," is still partly subjective and influenced by traditional assumptions, Howe said.

Paralympic athletes are divided into four main groups: amputees, the blind, those with cerebral palsy and those with spinal injuries or other physical disabilities. Depending on the sport, Paralympians either compete against others with the same disability, or across categories based on an evaluation of their ability to perform their chosen sport. Their classification is determined by a panel that includes experts in medicine and the sport's biomechanics.

"The goal is to make sure athletes in the same class have the same chance at a gold medal," said Peter Van de Vliet, science and medical director at the International Paralympic Committee.

He said they previously assessed athletes based on their disabilities but they now focus on their sporting potential, meaning athletes with different impairments can compete directly against each other.

"If you put a double leg amputee into a racing wheelchair and do the same for a paraplegic (who has non-functioning legs), they both are in the same position of needing to use their arms to propel the wheelchair," he said. "In that sense, classification is no different than age or weight categories."

The London Paralympics will once again include intellectually disabled athletes. Those events were suspended in the last two games after a scandal in which the winning basketball team from Spain was comprised mostly of players without any such disability. Van de Vliet said the classification for intellectually disabled competitors has been completely revamped since then and now includes stricter criteria, like a psychological evaluation.

Experts said the system is constantly evolving as officials get more experience with Paralympic sports.

Even the athletes admit the classification system isn't always straightforward.

British wheelchair sprinter Hannah Cockcroft, who is classified as having cerebral palsy and brain damage, has said skeptics sometimes wonder whether that's the right classification because she is so chatty. But she said it fits since two parts of her brain are damaged and she has trouble doing simple things like tying her shoes. Cockcroft said she occasionally wonders about the categories of other Paralympic athletes but without knowing their full story, it is impossible to judge.

Each of the 20 Paralympic sports has its own rules. For example, track and field divides athletes into categories based on their disability and its severity. Others, like judo and five-a-side soccer, are only open to blind athletes.

Swimming is one of a handful of sports that throws all of the athletes together, allowing amputees, those with cerebral palsy, spinal cord injuries and other disabilities to directly compete against each other.

That can make for a diverse line-up of athletes on the starting blocks, including swimmers with dwarfism, those missing a limb and others with cerebral palsy, who might look able-bodied but have coordination and movement problems. While the athletes with cerebral palsy have to work harder to coordinate their muscles to swim, those with dwarfism have a high level of drag in the water but may have better balance. In theory, the classification system is meant to even out those differences, Van de Vliet said.

"Classification is unique to Paralympic sport but it is crucial that the competition is fair and correct," Van de Vliet said. "For us, making sure athletes are in the correct class is as important as doping."